package com.hanxiaozhang.sourcecode.aqs;

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.*;

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic {@code int} value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated {@code int}
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 *
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 * <p>This class defines a nested {@link ConditionObject} class that
 * can be used as a {@link Condition} implementation by subclasses
 * supporting exclusive mode for which method {@link
 * #isHeldExclusively} reports whether synchronization is exclusively
 * held with respect to the current thread, method {@link #release}
 * invoked with the current {@link #getState} value fully releases
 * this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state.  No
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * <p>To use this class as the basis of a synchronizer, redefine the
 * following methods, as applicable, by inspecting and/or modifying
 * the synchronization state using {@link #getState}, {@link
 * #setState} and/or {@link #compareAndSetState}:
 *
 * <ul>
 * <li> {@link #tryAcquire}
 * <li> {@link #tryRelease}
 * <li> {@link #tryAcquireShared}
 * <li> {@link #tryReleaseShared}
 * <li> {@link #isHeldExclusively}
 * </ul>
 * <p>
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the <em>only</em> supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 *
 * <p>You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer.  You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 *
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 * <p>
 * (Shared mode is similar but may involve cascading signals.)
 *
 * <p id="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 * In particular, most fair synchronizers can define {@code tryAcquire}
 * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * {@code true}.  Other variations are possible.
 *
 * <p>Throughput and scalability are generally highest for the
 * default barging (also known as <em>greedy</em>,
 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * &quot;spin&quot; in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 *
 * <p>This class provides an efficient and scalable basis for
 * synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link java.util.concurrent.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 *
 * <h3>Usage Examples</h3>
 *
 * <p>Here is a non-reentrant mutual exclusion lock class that uses
 * the value zero to represent the unlocked state, and one to
 * represent the locked state. While a non-reentrant lock
 * does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor.
 * It also supports conditions and exposes
 * one of the instrumentation methods:
 *
 * <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Reports whether in locked state
 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 *
 *     // Acquires the lock if state is zero
 *     public boolean tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.currentThread());
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Releases the lock by setting state to zero
 *     protected boolean tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused
 *       if (getState() == 0) throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 *
 *     // Provides a Condition
 *     Condition newCondition() { return new ConditionObject(); }
 *
 *     // Deserializes properly
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
 *       s.defaultReadObject();
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *
 *   public void lock()                { sync.acquire(1); }
 *   public boolean tryLock()          { return sync.tryAcquire(1); }
 *   public void unlock()              { sync.release(1); }
 *   public Condition newCondition()   { return sync.newCondition(); }
 *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 *   public void lockInterruptibly() throws InterruptedException {
 *     sync.acquireInterruptibly(1);
 *   }
 *   public boolean tryLock(long timeout, TimeUnit unit)
 *       throws InterruptedException {
 *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *   }
 * }}</pre>
 *
 * <p>Here is a latch class that is like a
 * {@link java.util.concurrent.CountDownLatch CountDownLatch}
 * except that it only requires a single {@code signal} to
 * fire. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 *
 * <pre> {@code
 * class BooleanLatch {
 *
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     boolean isSignalled() { return getState() != 0; }
 *
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *
 *     protected boolean tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   public boolean isSignalled() { return sync.isSignalled(); }
 *   public void signal()         { sync.releaseShared(1); }
 *   public void await() throws InterruptedException {
 *     sync.acquireSharedInterruptibly(1);
 *   }
 * }}</pre>
 *
 * @author Doug Lea
 * @since 1.5
 */
public abstract class MyAbstractQueuedSynchronizer
        extends AbstractOwnableSynchronizer
        implements java.io.Serializable {

    private static final long serialVersionUID = 7373984972572414691L;

    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     * <p>
     * 创建实例，初始化时state是0
     */
    protected MyAbstractQueuedSynchronizer() {
    }

    // -------------------  Node start -------------------

    /**
     * CLH队列的节点
     *
     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
     * Hagersten) lock queue. CLH locks are normally used for
     * spinlocks.  We instead use them for blocking synchronizers, but
     * use the same basic tactic of holding some of the control
     * information about a thread in the predecessor of its node.  A
     * "status" field in each node keeps track of whether a thread
     * should block.  A node is signalled when its predecessor
     * releases.  Each node of the queue otherwise serves as a
     * specific-notification-style monitor holding a single waiting
     * thread. The status field does NOT control whether threads are
     * granted locks etc though.  A thread may try to acquire if it is
     * first in the queue. But being first does not guarantee success;
     * it only gives the right to contend.  So the currently released
     * contender thread may need to rewait.
     * <p>
     * 这个等待队列是“CLH”锁队列的变体。CLH锁通常用于旋转锁。
     *
     *
     * <p>To enqueue into a CLH lock, you atomically splice it in as new
     * tail. To dequeue, you just set the head field.
     * <pre>
     *      +------+  prev +-----+       +-----+
     * head |      | <---- |     | <---- |     |  tail
     *      +------+       +-----+       +-----+
     * </pre>
     *
     * <p>Insertion into a CLH queue requires only a single atomic
     * operation on "tail", so there is a simple atomic point of
     * demarcation from unqueued to queued. Similarly, dequeuing
     * involves only updating the "head". However, it takes a bit
     * more work for nodes to determine who their successors are,
     * in part to deal with possible cancellation due to timeouts
     * and interrupts.
     *
     * <p>The "prev" links (not used in original CLH locks), are mainly
     * needed to handle cancellation. If a node is cancelled, its
     * successor is (normally) relinked to a non-cancelled
     * predecessor. For explanation of similar mechanics in the case
     * of spin locks, see the papers by Scott and Scherer at
     * http://www.cs.rochester.edu/u/scott/synchronization/
     *
     * <p>We also use "next" links to implement blocking mechanics.
     * The thread id for each node is kept in its own node, so a
     * predecessor signals the next node to wake up by traversing
     * next link to determine which thread it is.  Determination of
     * successor must avoid races with newly queued nodes to set
     * the "next" fields of their predecessors.  This is solved
     * when necessary by checking backwards from the atomically
     * updated "tail" when a node's successor appears to be null.
     * (Or, said differently, the next-links are an optimization
     * so that we don't usually need a backward scan.)
     *
     * <p>Cancellation introduces some conservatism to the basic
     * algorithms.  Since we must poll for cancellation of other
     * nodes, we can miss noticing whether a cancelled node is
     * ahead or behind us. This is dealt with by always unparking
     * successors upon cancellation, allowing them to stabilize on
     * a new predecessor, unless we can identify an uncancelled
     * predecessor who will carry this responsibility.
     *
     * <p>CLH queues need a dummy header node to get started. But
     * we don't create them on construction, because it would be wasted
     * effort if there is never contention. Instead, the node
     * is constructed and head and tail pointers are set upon first
     * contention.
     *
     * <p>Threads waiting on Conditions use the same nodes, but
     * use an additional link. Conditions only need to link nodes
     * in simple (non-concurrent) linked queues because they are
     * only accessed when exclusively held.  Upon await, a node is
     * inserted into a condition queue.  Upon signal, the node is
     * transferred to the main queue.  A special value of status
     * field is used to mark which queue a node is on.
     *
     * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
     * Scherer and Michael Scott, along with members of JSR-166
     * expert group, for helpful ideas, discussions, and critiques
     * on the design of this class.
     */
    static final class Node {

        /**
         * Marker to indicate a node is waiting in shared mode
         * 标记 节点正在以共享模式等待
         */
        static final Node SHARED = new Node();

        /**
         * Marker to indicate a node is waiting in exclusive mode
         * 标记 节点正在以独占模式等待
         */
        static final Node EXCLUSIVE = null;

        /**
         * waitStatus value to indicate thread has cancelled
         * waitStatus的值：表示线程已取消
         */
        static final int CANCELLED = 1;

        /**
         * waitStatus value to indicate successor's thread needs unparking
         * waitStatus的值：表示后继线程需要unparking
         */
        static final int SIGNAL = -1;

        /**
         * waitStatus value to indicate thread is waiting on condition
         * waitStatus的值：表示线程在Condition队列上等待
         */
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should unconditionally propagate
         * waitStatus的值：表示无条件地被传播下
         */
        static final int PROPAGATE = -3;

        /**
         * Status field, taking on only the values:
         * SIGNAL: The successor of this node is (or will soon be) blocked (via park),
         * so the current node must unpark its successor when it releases or cancels.
         * To avoid races, acquire methods must first indicate they need a signal,
         * then retry the atomic acquire, and then,on failure, block.
         * <p>
         * SIGNAL（信号）: 此节点的后续节点通过park已被阻止或即将被阻止，所以，当前节点在后继节点释放或取消的时候，必须unpark。
         * 为了避免竞争，acquire()首先必须表明需要signal，然后再重试，通过原子性获取，并且在失败时阻塞。
         * <p>
         * CANCELLED:  This node is cancelled due to timeout or interrupt.
         * Nodes never leave this state. In particular,a thread with cancelled node never again blocks.
         * <p>
         * CANCELLED（取消）：节点的是取消状态，原因是超时或中断。进入取消状态的节点永远不会离开次状态。
         * 特别是，具有取消状态节点的线程不会再次阻塞。
         * <p>
         * CONDITION:  This node is currently on a condition queue.
         * It will not be used as a sync queue node until transferred, at which time the status will be set to 0.
         * (Use of this value here has nothing to do with the other uses of the field, but simplifies mechanics.)
         * <p>
         * CONDITION（条件）: 节点当前位于条件队列中。在转换之前，该节点不会用作同步队列的节点，此时status将设置为0
         * （这里使用该值与字段的其他用途无关，但是简化技术）
         * <p>
         * PROPAGATE:  A releaseShared should be propagated to other nodes. This is set (for head node only) in
         * doReleaseShared to ensure propagation continues, even if other operations have since intervened.
         * <p>
         * PROPAGATE（传播）：一个共享释放应该可以传播到其他节点。这在doReleaseShared中设置，
         * 但仅针对头部节点以确保传播继续。
         * <p>
         * 0:          None of the above
         * <p>
         * 0: 以上都没有
         * <p>
         * The values are arranged numerically to simplify use.
         * Non-negative values mean that a node doesn't need to signal.
         * So, most code doesn't need to check for particular values, just for sign.
         * <p>
         * 这些值以数字形式排列，以简化使用。非负值表示节点不需要发送信号。
         * 所以，大多数代码不需要检查特定的值，只需要检查符号（正负）。
         * <p>
         * The field is initialized to 0 for normal sync nodes, and CONDITION for condition nodes.
         * It is modified using CAS(or when possible, unconditional volatile writes).
         * <p>
         * 对于正常同步节点，字段初始化为0；对于条件节点，字段初始化为条件。
         * 使用CAS对其进行修改，或在可能的情况下，使用无条件易失性写入
         */
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueuing, and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         * <p>
         * 前继节点
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev's from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         * <p>
         * 后继节点
         */
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on construction and nulled out after use.
         * <p>
         * 节点的线程 构造时初始化，使用后为空
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special
         * value SHARED.  Because condition queues are accessed only
         * when holding in exclusive mode, we just need a simple
         * linked queue to hold nodes while they are waiting on
         * conditions. They are then transferred to the queue to
         * re-acquire. And because conditions can only be exclusive,
         * we save a field by using special value to indicate shared
         * mode.
         * <p>
         * 链接到下一个等待条件的节点，或是特殊值的SHARED节点。
         */
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode.
         * 如果节点正在共享模式下等待，则返回true
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null.
         * Use when predecessor cannot be null.  The null check could
         * be elided, but is present to help the VM.
         * <p>
         * 返回前继节点，如果为空，抛空指针
         *
         * @return the predecessor of this node
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null) {
                throw new NullPointerException();
            } else {
                return p;
            }

        }

        /**
         * Used to establish initial head or SHARED marker
         * 用于建立初始头部或共享标记
         */
        Node() {
        }

        /**
         * Used by addWaiter
         * 用于addWaiter
         *
         * @param thread
         * @param mode
         */
        Node(Thread thread, Node mode) {
            this.nextWaiter = mode;
            this.thread = thread;
        }

        /**
         * Used by Condition
         * 用于 Condition
         *
         * @param thread
         * @param waitStatus
         */
        Node(Thread thread, int waitStatus) {
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

    // -------------------  Node end -------------------


    /**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.
     * Note:If head exists, its waitStatus is guaranteed not to be CANCELLED.
     * <p>
     * 等待队列的头，延迟初始化。除初始化外，它仅通过方法setHead进行修改。
     * 注意：如果head存在，则保证其waitStatus不会被取消
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via method enq to add new wait node.
     * <p>
     * 等待队列的尾，延迟初始化。 仅通过enq方法修改以添加新的等待节点。
     */
    private transient volatile Node tail;

    /**
     * The synchronization state.
     * <p>
     * 同步状态
     */
    private volatile int state;

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a {@code volatile} read.
     *
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     *
     * @param newState the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read and write.
     * <p>
     * 当前状态值等于预期值值的时候，原子地将state设置为给定的更新状态
     * CAS
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     * value was not equal to the expected value.
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        // stateOffset ==>  unsafe.objectFieldOffset(MyAbstractQueuedSynchronizer.class.getDeclaredField("state"))
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    // Queuing utilities 排队实用程序

    /**
     * The number of nanoseconds for which it is faster to spin rather than to use timed park.
     * A rough estimate suffices to improve responsiveness with very short timeouts.
     * <p>
     * 自旋操作超时的阈值，自旋操作快于park的时间
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * <p>
     * 将节点插入队列，必要时进行初始化
     *
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(final Node node) {
        for (; ; ) {
            // tail赋值给t
            Node t = tail;
            // t 等于 null，必须先初始化头尾节点
            if (t == null) {
                // CAS 设置head，它是一个无状态的节点
                if (compareAndSetHead(new Node())) {
                    // head 赋值给 tail
                    tail = head;
                }
            } else {
                // node的前继节点赋值给t
                node.prev = t;
                // CAS 设置tail
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     * <p>
     * 为当前线程和入参给的模式（SHARED、EXCLUSIVE）创建节点并且排队
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        // 通过当前线程和节点模式创建节点
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure ？？
        // 将pred设置成tail节点
        Node pred = tail;
        // pred不为空，即tail不为空
        if (pred != null) {
            // 将node前继节点设置成pred
            node.prev = pred;
            // CAS替换Tail
            if (compareAndSetTail(pred, node)) {
                // pred的后继指向node
                pred.next = node;
                // 返回
                return node;
            }
        }
        // pred为空，即tail为空，需要初始化队列
        enq(node);
        return node;
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by acquire methods.
     * Also nulls out unused fields for sake of GC and to suppress unnecessary signals and traversals.
     * <p>
     * 将次节点设置为头结点，因此，退出队列。此方法只能acquire方法调用
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * Wakes up node's successor, if one exists.
     * <p>
     * 如果该节点存在后继节点，唤醒其后续节点
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try to clear in anticipation of signalling.
         * It is OK if this fails or if status is changed by waiting thread.
         * 如果状态为负（即，可能需要信号），则尝试在预期信号的情况下清除
         * 如果此操作失败或者等待线程更改了状态，则可以。
         */
        // 如果waitStatus小于0，CAS更新成0
        int ws = node.waitStatus;
        if (ws < 0) {
            compareAndSetWaitStatus(node, ws, 0);
        }

        /*
         * Thread to unpark is held in successor, which is normally just the next node.
         * But if cancelled or apparently null, traverse backwards from tail to find the actual
         * non-cancelled successor.
         *
         * 需要unpark的线程保存在后续节点中，通常只是下一个节点。
         * 但如果取消或明显为空，则从尾部向前遍历以找到实际的未取消的后继节点。
         */
        // 获取后继节点
        Node s = node.next;
        // s 为空 或者 waitStatus是CANCELLED状态
        if (s == null || s.waitStatus > 0) {
            s = null;
            // 从尾部向前遍历以找到实际的未取消的后继节点，找到离node最近后继节点
            for (Node t = tail; t != null && t != node; t = t.prev) {
                if (t.waitStatus <= 0) {
                    s = t;
                }
            }

        }
        //  如果s不为null，则唤醒s节点
        if (s != null) {
            LockSupport.unpark(s.thread);
        }

    }

    /**
     * Release action for shared mode -- signals successor and ensures propagation.
     * (Note: For exclusive mode, release just amounts to calling unparkSuccessor of head if it needs signal.)
     * <p>
     * 共享模式的释放操作 -- 发出信号给后继节点并确保传播。
     * 注意：对于独占模式，如果需要信号，release只相当于调用head的unparkSuccessor
     */
    private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (; ; ) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) {
                        continue;            // loop to recheck cases
                    }
                    unparkSuccessor(h);
                } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) {
                    continue;                // loop on failed CAS
                }

            }
            if (h == head) {
                break;  // loop if head changed
            }
        }
    }

    /**
     * Sets head of queue, and checks if successor may be waiting in shared mode,
     * if so propagating if either propagate > 0 or PROPAGATE status was set.
     * <p>
     * 设置队列头，并检查后续队列是否在共享模式下等待，
     * 如果是在设置了“propagate>0”或“传播状态”时，传播。
     *
     * @param node      the node
     * @param propagate the return value from a tryAcquireShared
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
                (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared()) {
                doReleaseShared();
            }
        }
    }

    // Utilities for various versions of acquire  acquire的各种版本的实用程序

    /**
     * Cancels an ongoing attempt to acquire.
     * <p>
     * 取消正在进行的获Acquire
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist  如果节点不存在，则忽略
        if (node == null) {
            return;
        }
        // 当前节点设置为null
        node.thread = null;

        // Skip cancelled predecessors 跳过取消的前继节点
        Node pred = node.prev;
        while (pred.waitStatus > 0) {
            node.prev = pred = pred.prev;
        }


        // predNext is the apparent node to unsplice.
        // CASes below will fail if not, in which case, we lost race vs another cancel or signal,
        // so no further action is necessary.
        // 获取前继节点的后继节点
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        // 将节点的waitStatus设置为CANCELLED
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves. 如果我们是tail，就把自己移走。
        // node是tail 并且 CAS替换tail
        if (node == tail && compareAndSetTail(node, pred)) {
            // CAS将tail的后继设置为空，即移除当前pred的所有后继节点(node节点和node节点之前连续是CANCELLED状态的节点)
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link so it will get one.
            // Otherwise wake it up to propagate.
            // 如果后继者需要pred节点状态是SIGNAL，则尝试设置pred的next节点的链接，这样它就会得到一个链接，否则唤醒它传播
            int ws;
            // i.pred不是尾结点，又不是头结点
            // ii. pred.waitStatus是SIGNAL 或者 ws <= 0时，可以通过CAS可以替换成SIGNAL
            // iii. pred.thread不是null
            if (pred != head &&
                    ((ws = pred.waitStatus) == Node.SIGNAL || (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                    pred.thread != null) {
                // 获取当前节点后继节点
                Node next = node.next;
                // next不为空，并且next.waitStatus不是CANCELLED
                if (next != null && next.waitStatus <= 0) {
                    // pred节点的next从predNext修改成next，这里也会node节点和node节点之前连续是CANCELLED状态的节点
                    compareAndSetNext(pred, predNext, next);
                }
            } else {
                // 如果pred节点无法提供给node的后继节点SIGNAL，则直接唤醒node的后继节点
                unparkSuccessor(node);
            }
            // help GC
            node.next = node;
        }
    }

    /**
     * Checks and updates status for a node that failed to acquire.Returns true if thread should block.
     * This is the main signal control in all acquire loops.  Requires that pred == node.prev.
     * <p>
     * 检查并更新"获取失败节点"的状态。如果返回true，则线程需要阻塞，即当前节点，要调用part。
     * 这是所有采集回路中的主要信号控制。要求pred == node.prev。
     * <p>
     * 思路：
     * 如果前继节点的waitStatus是SIGNAL，返回true；
     * 如果前继节点的waitStatus是CANCELLED，找到不是CANCELLED节点为止，返回false；
     * 如果前继节点的waitStatus是0或PROPAGATE，将其设置为SIGNAL，返回false；
     *
     * @param pred node's predecessor holding status
     * @param node the node
     * @return {@code true} if thread should block
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        // 前继节点的等待状态赋值给ws
        int ws = pred.waitStatus;
        // 如果ws等于 Node.SIGNAL
        if (ws == Node.SIGNAL) {
            /*
             * This node has already set status asking a release to signal it, so it can safely park.
             * 此节点已经设置了请求释放的状态以向其发送信号，因此它可以安全park。
             */
            return true;
        }
        // ws >0 即，是CANCELLED
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and indicate retry.
             * 前继被取消了。跳过所有前继被取消的节点。
             */
            do {
                // i. pred = pred.prev
                // ii. node.prev = pred
                node.prev = pred = pred.prev;
                // 循环条件pred.waitStatus > 0
            } while (pred.waitStatus > 0);
            // 将不是取消状态的前继节点的后继设置为node
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.
             * Indicate that we need a signal, but don't park yet. ??
             * Caller will need to retry to make sure it cannot acquire before parking. ??
             *
             * waitStatus 必须是 0 or PROPAGATE
             * 使用CAS将pred设置为SIGNAL
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * Convenience method to interrupt current thread.
     * <p>
     * 中断当前线程的简便方法
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
     * Convenience method to park and then check if interrupted
     * <p>
     * park()，然后检查是否中断
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        // 调用 LockSupport.park(this) 线程就阻塞在这里了
        LockSupport.park(this);
        return Thread.interrupted();
    }

    /*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     *
     * 各种的获取方式，在独占/共享和控制模式各不相同，基本都是一样的。
     * 但是，异常机制（包括确保在tryAcquire抛出异常时取消）和其他控件的交互，会导致不一样，
     * 但不会对性能造成太大的损害。
     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in queue.
     * Used by condition wait methods as well as acquire.
     * <p>
     * 已在队列中的线程以独占不间断模式获取。
     * 用于条件等待方法和获取
     *
     * @param node the node
     * @param arg  the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        // 失败标识
        boolean failed = true;
        try {
            // 中断标识
            boolean interrupted = false;
            // 无条件死循环
            for (; ; ) {
                // 获取前继节点赋值给p
                final Node p = node.predecessor();
                // 前继节点是head，并且tryAcquire(arg)成功，证明获取成功
                if (p == head && tryAcquire(arg)) {
                    // 设置head
                    setHead(node);
                    // help GC
                    p.next = null;
                    failed = false;
                    return interrupted;
                }
                //  shouldParkAfterFailedAcquire(p, node) 返回true，调用parkAndCheckInterrupt()
                // 即，当前线程阻塞，调用的part()
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt()) {
                    interrupted = true;
                }
            }
        } finally {
            // 出现异常
            if (failed) {
                // 取消正在进行的Acquire
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in exclusive interruptible mode.
     * <p>
     * 以独占可中断模式获取
     *
     * @param arg the acquire argument
     */
    private void doAcquireInterruptibly(int arg)
            throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (; ; ) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt()) {
                    throw new InterruptedException();
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in exclusive timed mode.
     * <p>
     * 以独占定时模式获取。
     *
     * @param arg          the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L) {
            return false;
        }
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (; ; ) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L) {
                    return false;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }
                if (Thread.interrupted()) {
                    throw new InterruptedException();
                }

            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in shared uninterruptible mode.
     * <p>
     * 以共享不间断模式获取。
     *
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (; ; ) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted) {
                            selfInterrupt();
                        }
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt()) {
                    interrupted = true;
                }

            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in shared interruptible mode.
     * <p>
     * 以共享可中断模式获取。
     *
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
            throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (; ; ) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt()) {
                    throw new InterruptedException();
                }
            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    /**
     * Acquires in shared timed mode.
     * <p>
     * 以共享定时模式获取。
     *
     * @param arg          the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L) {
            return false;
        }
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (; ; ) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L) {
                    return false;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }

                if (Thread.interrupted()) {
                    throw new InterruptedException();
                }

            }
        } finally {
            if (failed) {
                cancelAcquire(node);
            }
        }
    }

    // Main exported methods 主要的对外方法

    /**
     * Attempts to acquire in exclusive mode. This method should query if the state of the object permits
     * it to be acquired in the exclusive mode, and if so to acquire it.
     * <p>
     * 尝试以独占模式获取。此方法应查询对象的状态是否允许以独占模式获取该对象，如果允许，则获取该对象。
     *
     * <p>This method is always invoked by the thread performing acquire.
     * If this method reports failure, the acquire method may queue the thread,
     * if it is not already queued, until it is signalled by a release from some other thread.
     * This can be used to implement method {@link Lock#tryLock()}.
     * <p>
     * 执行获取的线程始终调用此方法。
     * 如果此方法报告失败，则acquire()可能会将线程排队，如果它尚未排队，则会让它排队，直到其它线程发出释放信号。
     * 这可用于实现方法Lock.tryLock()。
     *
     *
     * <p>The default
     * implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *            passed to an acquire method, or is the value saved on entry
     *            to a condition wait.  The value is otherwise uninterpreted
     *            and can represent anything you like.
     * @return {@code true} if successful. Upon success, this object has
     * been acquired.
     * @throws IllegalMonitorStateException  if acquiring would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in exclusive mode.
     * <p>
     * 尝试设置状态在独占模式下释放
     * <p>
     * This method is always invoked by the thread performing release.
     * <p>
     * 执行释放的线程总是调用此方法。
     *
     * <p>The default implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *            passed to a release method, or the current state value upon
     *            entry to a condition wait.  The value is otherwise
     *            uninterpreted and can represent anything you like.
     * @return {@code true} if this object is now in a fully released
     * state, so that any waiting threads may attempt to acquire;
     * and {@code false} otherwise.
     * @throws IllegalMonitorStateException  if releasing would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared mode, and if so to acquire it.
     * <p>
     * 尝试在共享模式下获取。此方法应查询对象的状态是否允许在共享模式下获取该对象，以及是否允许获取该对象。
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *            passed to an acquire method, or is the value saved on entry
     *            to a condition wait.  The value is otherwise uninterpreted
     *            and can represent anything you like.
     * @return a negative value on failure; zero if acquisition in shared
     * mode succeeded but no subsequent shared-mode acquire can
     * succeed; and a positive value if acquisition in shared
     * mode succeeded and subsequent shared-mode acquires might
     * also succeed, in which case a subsequent waiting thread
     * must check availability. (Support for three different
     * return values enables this method to be used in contexts
     * where acquires only sometimes act exclusively.)  Upon
     * success, this object has been acquired.
     * @throws IllegalMonitorStateException  if acquiring would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in shared mode.
     * <p>
     * 尝试设置状态在共享模式下释放
     * <p>
     * This method is always invoked by the thread performing release.
     * <p>
     * 执行释放的线程总是调用此方法。
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *            passed to a release method, or the current state value upon
     *            entry to a condition wait.  The value is otherwise
     *            uninterpreted and can represent anything you like.
     * @return {@code true} if this release of shared mode may permit a
     * waiting acquire (shared or exclusive) to succeed; and
     * {@code false} otherwise
     * @throws IllegalMonitorStateException  if releasing would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Returns {@code true} if synchronization is held exclusively with respect to the current (calling) thread.
     * This method is invoked upon each call to a non-waiting {@link ConditionObject} method.
     * (Waiting methods instead invoke {@link #release}.)
     * <p>
     * 如果同步以独占方式进行的当前（调用）线程进行，则返回true。
     * 每次调用非等待的{@link ConditionObject}方法时都会调用此方法
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     * This method is invoked internally only within {@link ConditionObject} methods, so need
     * not be defined if conditions are not used.
     * <p>
     * 此方法仅在{@link ConditionObject}方法内部调用，因此如果不使用条件，则无需定义。
     *
     * @return {@code true} if synchronization is held exclusively;
     * {@code false} otherwise
     * @throws UnsupportedOperationException if conditions are not supported
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented by invoking at least once {@link #tryAcquire},returning on success.
     * Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking {@link #tryAcquire} until success.
     * This method can be used to implement method {@link Lock#lock}.
     * <p>
     * 以独占模式获取，忽略中断。通过调用至少一次tryAcquire来实现，并在成功时返回。
     * 否则线程将排队，可能反复阻塞和解锁,直到调用tryAcquire成功。
     * 此方法可用于实现方法Lock
     *
     * @param arg the acquire argument.  参数，这个参数直接传递给tryAcquire()，具体含义看继承类如何定义
     *            This value is conveyed to {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     */
    public final void acquire(int arg) {

        // i.tryAcquire(arg) 失败
        // ii.addWaiter(Node.EXCLUSIVE) 为当前线程创建独享节点，并且放入同步队列
        // iii.acquireQueued(node,arg)
        if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) {
            // 中断当前线程
            selfInterrupt();
        }
    }

    /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking at least once {@link #tryAcquire}, returning on success.
     * Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.
     * This method can be used to implement method {@link Lock#lockInterruptibly}.
     * <p>
     * 以独占模式获取，可以中断将中止。
     * 通过首先检查中断状态，然后至少调用一次tryAcquire，并在成功时返回来实现。
     * 否则线程将排队，可能会重复阻塞和取消阻塞，调用 tryAcquire，直到成功或线程中断。
     * 此方法可用于实现方法Lock.lockInterruptibly。
     *
     * @param arg the acquire argument.  This value is conveyed to
     *            {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }
        if (!tryAcquire(arg)) {
            doAcquireInterruptibly(arg);
        }

    }

    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted, and failing if the given timeout elapses.
     * Implemented by first checking interrupt status, then invoking at least once {@link #tryAcquire}, returning on success.
     * Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted or the timeout elapses.
     * This method can be used to implement method {@link Lock#tryLock(long, TimeUnit)}.
     * <p>
     * 尝试以独占模式获取，如果中断则中止，如果给定的超时时间过去则失败。
     * 通过首先检查中断状态，然后至少调用一次tryAcquire，并在成功时返回来实现。
     * 否则，线程将排队，可能会继续阻塞和取消阻塞，调用tryAcquire，直到成功或线程中断或超时。
     * 此方法可用于实现方法Lock.tryLock（long，TimeUnit）。
     *
     * @param arg          the acquire argument.  This value is conveyed to
     *                     {@link #tryAcquire} but is otherwise uninterpreted and
     *                     can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }
        return tryAcquire(arg) ||
                doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * Releases in exclusive mode.  Implemented by unblocking one or more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     * <p>
     * 以独占模式释放。如果tryRelease返回true，则通过取消阻止一个或多个线程来实现。
     *
     * @param arg the release argument.
     *            This value is conveyed to {@link #tryRelease} but is otherwise uninterpreted and
     *            can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {

        if (tryRelease(arg)) {
            Node h = head;
            // h不为空，h.waitStatus != 0，唤醒头结点
            if (h != null && h.waitStatus != 0) {
                unparkSuccessor(h);
            }
            return true;
        }
        return false;
    }

    /**
     * Acquires in shared mode, ignoring interrupts.  Implemented by first invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking {@link
     * #tryAcquireShared} until success.
     * <p>
     * 在共享模式下获取，忽略中断。通过至少调用一次tryAcquireShared来实现，并在成功时返回。
     *
     * @param arg the acquire argument.  This value is conveyed to
     *            {@link #tryAcquireShared} but is otherwise uninterpreted
     *            and can represent anything you like.
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0) {
            doAcquireShared(arg);
        }
    }

    /**
     * Acquires in shared mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread is interrupted.
     * <p>
     * 在共享模式下获取，如果中断将中止。
     * 通过首先检查中断状态，然后至少调用一次tryAcquireShared，在成功时返回来实现。
     * 否则线程将排队，可能会重复阻塞和取消阻塞，调用tryAcquireShared，直到成功或线程中断。
     *
     * @param arg the acquire argument.
     *            This value is conveyed to {@link #tryAcquireShared} but is
     *            otherwise uninterpreted and can represent anything
     *            you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }
        if (tryAcquireShared(arg) < 0) {
            doAcquireSharedInterruptibly(arg);
        }

    }

    /**
     * Attempts to acquire in shared mode, aborting if interrupted, and failing if the given timeout elapses.
     * Implemented by first checking interrupt status, then invoking at least once {@link #tryAcquireShared},
     * returning on success.
     * Otherwise, the thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread is interrupted or the timeout elapses.
     * <p>
     * 在共享模式下尝试获取，如果中断将中止，如果超过给定的超时时间，则失败。
     * 首先通过检查中断状态，然后至少调用一次tryAcquireShared，在成功时返回来实现。
     * 否则，线程将排队，可能会重复阻塞和取消阻塞，调用tryAcquireShared，直到成功或线程中断或超时
     *
     * @param arg          the acquire argument.  This value is conveyed to
     *                     {@link #tryAcquireShared} but is otherwise uninterpreted
     *                     and can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }

        return tryAcquireShared(arg) >= 0 ||
                doAcquireSharedNanos(arg, nanosTimeout);
    }

    /**
     * Releases in shared mode.
     * Implemented by unblocking one or more threads if {@link #tryReleaseShared} returns true.
     * <p>
     * 在共享模式下释放
     * 如果tryReleaseShared返回true，则通过取消阻止一个或多个线程来实现。
     *
     * @param arg the release argument.  This value is conveyed to
     *            {@link #tryReleaseShared} but is otherwise uninterpreted
     *            and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods 排队检查方法

    /**
     * Queries whether any threads are waiting to acquire.
     * Note that because cancellations due to interrupts and timeouts may occur at any time,
     * a {@code true} return does not guarantee that any other thread will ever acquire.
     * <p>
     * 查询是否有线程正在等待获取。
     * 请注意，由于中断和超时导致的取消可能随时发生，返回true并不保证任何其他线程都会获得。
     * <p>
     * In this implementation, this operation returns in constant time.
     * <p>
     * 在此实现中，此操作以恒定时间返回。
     *
     * @return {@code true} if there may be other threads waiting to acquire
     */
    public final boolean hasQueuedThreads() {
        return head != tail;
    }

    /**
     * Queries whether any threads have ever contended to acquire this synchronizer;
     * that is if an acquire method has ever blocked.
     * <p>
     * 查询是否有线程争用过此同步器；也就是说，如果acquire方法曾经被阻止过
     * <p>
     * In this implementation, this operation returns in constant time.
     * <p>
     * 在此实现中，此操作以恒定时间返回。
     *
     * @return {@code true} if there has ever been contention
     */
    public final boolean hasContended() {
        return head != null;
    }

    /**
     * Returns the first (longest-waiting) thread in the queue, or {@code null} if no threads are currently queued.
     * <p>
     * 返回队列中的第一个（等待时间最长）线程，如果当前没有线程排队，则返回{@code null}。
     *
     * <p>In this implementation, this operation normally returns in constant time,
     * but may iterate upon contention if other threads are concurrently modifying the queue.
     * <p>
     * 在此实现中，此操作通常以恒定时间返回，但如果其他线程同时修改队列，则可能会在争用时迭代。
     *
     * @return the first (longest-waiting) thread in the queue, or {@code null} if no threads are currently queued
     */
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }

    /**
     * Version of getFirstQueuedThread called when fastpath fails
     * <p>
     * 当fast path失败时,getFirstQueuedThread会调用
     */
    private Thread fullGetFirstQueuedThread() {
        /*
         * The first node is normally head.next. Try to get its
         * thread field, ensuring consistent reads: If thread
         * field is nulled out or s.prev is no longer head, then
         * some other thread(s) concurrently performed setHead in
         * between some of our reads. We try this twice before
         * resorting to traversal.
         */
        Node h, s;
        Thread st;
        if (((h = head) != null && (s = h.next) != null &&
                s.prev == head && (st = s.thread) != null) ||
                ((h = head) != null && (s = h.next) != null &&
                        s.prev == head && (st = s.thread) != null)) {
            return st;
        }


        /*
         * Head's next field might not have been set yet, or may have
         * been unset after setHead. So we must check to see if tail
         * is actually first node. If not, we continue on, safely
         * traversing from tail back to head to find first,
         * guaranteeing termination.
         */

        Node t = tail;
        Thread firstThread = null;
        while (t != null && t != head) {
            Thread tt = t.thread;
            if (tt != null) {
                firstThread = tt;
            }
            t = t.prev;
        }
        return firstThread;
    }

    /**
     * Returns true if the given thread is currently queued.
     * <p>
     * 如果给定线程当前已排队，则返回true。
     * <p>
     * This implementation traverses the queue to determine presence of the given thread.
     * <p>
     * 此实现遍历队列以确定给定线程的存在。
     *
     * @param thread the thread
     * @return {@code true} if the given thread is on the queue
     * @throws NullPointerException if the thread is null
     */
    public final boolean isQueued(Thread thread) {
        if (thread == null) {
            throw new NullPointerException();
        }
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread == thread) {
                return true;
            }
        }

        return false;
    }

    /**
     * Returns {@code true} if the apparent first queued thread, if one exists, is waiting in exclusive mode.
     * If this method returns {@code true}, and the current thread is attempting to acquire in shared mode
     * (that is, this method is invoked from {@link #tryAcquireShared})
     * then it is guaranteed that the current thread is not the first queued thread.
     * <p>
     * Used only as a heuristic in ReentrantReadWriteLock.
     * <p>
     * 如果存在第一个排队线程以独占模式等待，则返回true。如果此方法返回true，并且当前线程正在尝试以共享模式获取。
     * 即，此方法从tryAcquireShared调用，然后保证当前线程不是第一个排队的线程。
     * <p>
     * 仅在ReentrantReadWriteLock中使用
     */
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null &&
                (s = h.next) != null &&
                !s.isShared() &&
                s.thread != null;
    }

    /**
     * Queries whether any threads have been waiting to acquire longer than the current thread.
     * <p>
     * 查询是否有线程等待获取的时间长于当前线程
     * <p>
     * An invocation of this method is equivalent to (but may be more efficient than):
     * <pre> {@code getFirstQueuedThread() != Thread.currentThread() && hasQueuedThreads()}</pre>
     * <p>
     * 调用此方法相当于（但可能比）：
     * getFirstQueuedThread() != Thread.currentThread() && hasQueuedThreads()
     * <p>
     * Note that because cancellations due to interrupts and timeouts may occur at any time,
     * a {@code true} return does not guarantee that some other thread will acquire before the current thread.
     * Likewise, it is possible for another thread to win a race to enqueue after this method has returned {@code false},
     * due to the queue being empty.
     * <p>
     * 请注意，由于中断和超时导致的取消可能随时发生，返回true并不保证其他线程将在当前线程之前获取。
     * 同样，由于队列为空，因此在该方法返回false后，另一个线程也可能赢得排队比赛。
     *
     * <p>This method is designed to be used by a fair synchronizer to avoid
     * <a href="AbstractQueuedSynchronizer#barging">barging</a>.
     * <p>
     * Such a synchronizer's {@link #tryAcquire} method should return {@code false},
     * and its {@link #tryAcquireShared} method should return a negative value,
     * if this method returns {@code true} (unless this is a reentrant acquire).
     * For example, the {@code tryAcquire} method for a fair, reentrant, exclusive mode
     * synchronizer might look like this:
     * <p>
     * 这种同步器的tryAcquire方法应该返回false，如果此方法返回true（除非这是可重入获取），
     * 则其tryAcquireShared方法应该返回负值。
     * 例如，公平、可重入、独占模式同步器的tryAcquire方法可能如下所示：
     *
     * <pre> {@code
     * protected boolean tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}</pre>
     *
     * @return {@code true} if there is a queued thread preceding the current thread,
     * and {@code false} if the current thread is at the head of the queue or the queue is empty
     * @since 1.7
     */
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        // Read fields in reverse initialization order
        Node t = tail;
        Node h = head;
        Node s;
        return h != t &&
                ((s = h.next) == null || s.thread != Thread.currentThread());
    }


    // Instrumentation and monitoring methods 仪器和监测方法

    /**
     * Returns an estimate of the number of threads waiting to acquire.
     * The value is only an estimate because the number of threads may change dynamically
     * while this method traverses internal data structures.
     * This method is designed for use in monitoring system state, not for synchronization control.
     * <p>
     * 返回等待获取的线程数的估计值。
     * 该值只是一个估计值，因为当此方法遍历内部数据结构时，线程的数量可能会动态变化。
     * 此方法设计用于监视系统状态，而不是用于同步控制。
     *
     * @return the estimated number of threads waiting to acquire
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null) {
                ++n;
            }
        }
        return n;
    }

    /**
     * Returns a collection containing threads that may be waiting to acquire.
     * Because the actual set of threads may change dynamically while constructing this result,
     * the returned collection is only a best-effort estimate.
     * The elements of the returned collection are in no particular order.
     * This method is designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     * <p>
     * 返回包含可能正在等待获取的线程的集合。
     * 由于实际的线程集在构造此结果时可能会动态更改，返回的集合只是一个尽力而为的估计。
     * 返回集合的元素没有特定的顺序。
     * 此方法旨在促进提供更广泛监控设施的子类的构建。
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null) {
                list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to acquire in exclusive mode.
     * This has the same properties as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     * <p>
     * 返回一个集合，其中包含可能正在以独占模式等待获取的线程。
     * 它与getQueuedThreads具有相同的属性，只是它只返回由于独占获取而等待的线程。
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t != null) {
                    list.add(t);
                }
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to acquire in shared mode.
     * This has the same properties as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     * <p>
     * 返回一个集合，其中包含可能正在共享模式下等待获取的线程
     * 它与getQueuedThreads具有相同的属性，只是它只返回由于共享获取而等待的线程。
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t != null) {
                    list.add(t);
                }
            }
        }
        return list;
    }

    /**
     * Returns a string identifying this synchronizer, as well as its state.
     * The state, in brackets, includes the String {@code "State ="} followed by the
     * current value of {@link #getState}, and either {@code "nonempty"} or {@code "empty"} depending on whether the
     * queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state
     */
    @Override
    public String toString() {
        int s = getState();
        String q = hasQueuedThreads() ? "non" : "";
        return super.toString() +
                "[State = " + s + ", " + q + "empty queue]";
    }


    // Internal support methods for Conditions 条件的内部支持方法

    /**
     * Returns true if a node, always one that was initially placed on a condition queue,
     * is now waiting to reacquire on sync queue.
     * <p>
     * 如果一个节点始终是最初放置在条件队列中的节点，现在正在等待重新获取同步队列，则返回true。
     *
     * @param node the node
     * @return true if is reacquiring
     */
    final boolean isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev == null) {
            return false;
        }
        if (node.next != null) {
            // If has successor, it must be on queue
            return true;
        }

        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * <p>
     * 通过从尾部向后搜索，如果节点位于同步队列上，则返回true
     * 仅在isOnSyncQueue需要时调用
     *
     * @return true if present
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (; ; ) {
            if (t == node) {
                return true;
            }
            if (t == null) {
                return false;
            }
            t = t.prev;
        }
    }

    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     * <p>
     * 将节点从条件队列传输到同步队列。如果成功，则返回true。
     *
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            return false;
        }

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) {
            LockSupport.unpark(node.thread);
        }
        return true;
    }

    /**
     * Transfers node, if necessary, to sync queue after a cancelled wait.
     * Returns true if thread was cancelled before being signalled.
     * <p>
     * 如有必要，在取消等待后将节点传输到同步队列。如果线程在发出信号之前被取消，则返回true
     *
     * @param node the node
     * @return true if cancelled before the node was signalled
     */
    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while (!isOnSyncQueue(node)) {
            Thread.yield();
        }
        return false;
    }

    /**
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * <p>
     * 使用当前状态的值调用release；返回保存状态。取消节点并在失败时引发异常。
     *
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            if (failed) {
                node.waitStatus = Node.CANCELLED;
            }
        }
    }

    // Instrumentation methods for conditions 测量conditions的仪器方法

    /**
     * Queries whether the given ConditionObject uses this synchronizer as its lock.
     * <p>
     * 查询给定的ConditionObject是否将此同步器用作其锁。
     *
     * @param condition the condition
     * @return {@code true} if owned
     * @throws NullPointerException if the condition is null
     */
    public final boolean owns(ConditionObject condition) {
        return condition.isOwnedBy(this);
    }

    /**
     * Queries whether any threads are waiting on the given condition associated with this synchronizer.
     * Note that because timeouts and interrupts may occur at any time,
     * a {@code true} return does not guarantee that a future {@code signal} will awaken any threads.
     * This method is designed primarily for use in monitoring of the system state.
     * <p>
     * 查询是否有线程正在等待与此同步器关联的给定条件
     * 请注意，由于超时和中断可能随时发生，返回true并不保证将来的signal将唤醒任何线程。
     * 此方法主要用于监视系统状态。
     *
     * @param condition the condition
     * @return {@code true} if there are any waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *                                      is not held
     * @throws IllegalArgumentException     if the given condition is
     *                                      not associated with this synchronizer
     * @throws NullPointerException         if the condition is null
     */
    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition)) {
            throw new IllegalArgumentException("Not owner");
        }
        return condition.hasWaiters();
    }

    /**
     * Returns an estimate of the number of threads waiting on the given condition associated with this synchronizer.
     * Note that because timeouts and interrupts may occur at any time,
     * the estimate serves only as an upper bound on the actual number of waiters.
     * This method is designed for use in monitoring of the system state, not for synchronization control.
     * <p>
     * 返回等待与此同步器关联的给定条件的线程数的估计值。
     * 请注意，由于超时和中断可能随时发生，估计值仅作为实际服务员人数的上限。
     * 此方法设计用于监视系统状态，而不是用于同步控制
     *
     * @param condition the condition
     * @return the estimated number of waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *                                      is not held
     * @throws IllegalArgumentException     if the given condition is
     *                                      not associated with this synchronizer
     * @throws NullPointerException         if the condition is null
     */
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition)) {
            throw new IllegalArgumentException("Not owner");
        }
        return condition.getWaitQueueLength();
    }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this synchronizer.
     * Because the actual set of threads may change dynamically while constructing this result,
     * the returned collection is only a best-effort estimate.
     * The elements of the returned collection are in no particular order.
     * <p>
     * 返回一个集合，其中包含可能等待与此同步器关联的给定条件的线程。
     * 由于实际的线程集在构造此结果时可能会动态更改，返回的集合只是一个尽力而为的估计。
     * 返回集合的元素没有特定的顺序。
     *
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *                                      is not held
     * @throws IllegalArgumentException     if the given condition is
     *                                      not associated with this synchronizer
     * @throws NullPointerException         if the condition is null
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition)) {
            throw new IllegalArgumentException("Not owner");
        }
        return condition.getWaitingThreads();
    }

    /**
     * Condition implementation for a {@link MyAbstractQueuedSynchronizer}
     * serving as the basis of a {@link Lock} implementation.
     * <p>
     * 作为Lock实现基础的AbstractQueuedSynchronizer的条件实现
     * <p>
     * Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock and Condition users.
     * Exported versions of this class will in general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated {@code AbstractQueuedSynchronizer}.
     * <p>
     * 此类的方法文档描述的是机制，而不是锁和条件用户的行为规范。
     * 该类的导出版本通常需要随附描述条件语义的文档，这些语义依赖于关联的AbstractQueuedSynchronizer的语义。
     * <p>
     * This class is Serializable, but all fields are transient, so deserialized conditions have no waiters.
     * <p>
     * 此类是可序列化的，但所有字段都是暂时的，因此反序列化的条件没有等待者。
     */
    public class ConditionObject implements Condition, java.io.Serializable {

        private static final long serialVersionUID = 1173984872572414699L;

        /**
         * First node of condition queue.
         * <p>
         * 条件队列的第一个节点
         */
        private transient Node firstWaiter;

        /**
         * Last node of condition queue.
         * <p>
         * 条件队列的最后一个节点
         */
        private transient Node lastWaiter;

        /**
         * Creates a new {@code ConditionObject} instance.
         */
        public ConditionObject() {
        }

        // Internal methods 内部方法

        /**
         * Adds a new waiter to wait queue.
         * <p>
         * 将新的waiter添加到等待队列
         *
         * @return its new wait node
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null) {
                firstWaiter = node;
            } else {
                t.nextWaiter = node;
            }

            lastWaiter = node;
            return node;
        }

        /**
         * Removes and transfers nodes until hit non-cancelled one or null.
         * Split out from signal in part to encourage compilers to inline the case of no waiters.
         * <p>
         * 删除并且传输节点，直到命中 non-cancelled one或null。
         * 从信号中分离出来，部分是为了鼓励编译器在没有等待者的情况下内联
         *
         * @param first (non-null) the first node on condition queue
         */
        private void doSignal(Node first) {
            do {
                if ((firstWaiter = first.nextWaiter) == null) {
                    lastWaiter = null;
                }
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                    (first = firstWaiter) != null);
        }

        /**
         * Removes and transfers all nodes.
         * <p>
         * 删除并且传输全部节点
         *
         * @param first (non-null) the first node on condition queue
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        /**
         * Unlinks cancelled waiter nodes from condition queue. Called only while holding lock.
         * This is called when cancellation occurred during condition wait,
         * and upon insertion of a new waiter when lastWaiter is seen to have been cancelled.
         * This method is needed to avoid garbage retention in the absence of signals.
         * So even though it may require a full traversal,
         * it comes into play only when timeouts or cancellations occur in the absence of signals.
         * It traverses all nodes rather than stopping at a particular target  to unlink all
         * pointers to garbage nodes without requiring many re-traversals during cancellation storms.
         * <p>
         * 从条件队列中取消已取消的waiter节点的链接。仅在持有锁时调用。
         * 当在条件等待期间发生取消时，以及在看到lastWaiter已被取消时插入新服务员时，调用此函数。
         * 需要这种方法来避免在没有信号的情况下垃圾保留。
         * 因此，即使它可能需要一次完整的遍历，只有在没有信号的情况下发生超时或取消时，它才会起作用。
         * 它遍历所有节点，而不是在特定目标处停止，以取消所有指针到垃圾节点的链接，
         * 而无需在取消风暴期间多次重新遍历。
         */
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null) {
                        firstWaiter = next;
                    } else {
                        trail.nextWaiter = next;
                    }
                    if (next == null) {
                        lastWaiter = trail;
                    }
                } else {
                    trail = t;
                }
                t = next;
            }
        }

        // public methods 公共方法

        /**
         * Moves the longest-waiting thread, if one exists,
         * from the wait queue for this condition to the wait queue for the owning lock.
         * <p>
         * 将等待时间最长的线程（如果存在）从该条件的等待队列移动到拥有锁的等待队列。
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        @Override
        public final void signal() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }
            Node first = firstWaiter;
            if (first != null) {
                doSignal(first);
            }
        }

        /**
         * Moves all threads from the wait queue for this condition to
         * the wait queue for the owning lock.
         * <p>
         * 将此条件的所有线程从等待队列移动到拥有锁的等待队列。
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        @Override
        public final void signalAll() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }
            Node first = firstWaiter;
            if (first != null) {
                doSignalAll(first);
            }
        }

        /**
         * Implements uninterruptible condition wait.
         * <p>
         * 实现不间断条件等待。
         *
         * <ol>
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled.
         * <li> Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * </ol>
         */
        @Override
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted()) {
                    interrupted = true;
                }

            }
            if (acquireQueued(node, savedState) || interrupted) {
                selfInterrupt();
            }

        }

        /*
         * For interruptible waits, we need to track whether to throw InterruptedException,
         * if interrupted while blocked on condition,versus reinterrupt current thread,
         * if interrupted while blocked waiting to re-acquire.
         *
         * 对于可中断的等待，我们需要跟踪是否抛出InterruptedException，
         * 如果在条件阻塞时中断，而不是重新中断当前线程，
         * 如果在阻塞等待重新获取时中断。
         */

        /**
         * Mode meaning to reinterrupt on exit from wait
         * <p>
         * 退出等待时重新中断
         */
        private static final int REINTERRUPT = 1;

        /**
         * Mode meaning to throw InterruptedException on exit from wait
         * <p>
         * 退出等待时抛出InterruptedException
         */
        private static final int THROW_IE = -1;

        /**
         * Checks for interrupt, returning THROW_IE if interrupted before signalled,
         * REINTERRUPT if after signalled, or 0 if not interrupted.
         * <p>
         * 检查中断：如果在发出信号之前中断，则返回抛出，如果在发出信号之后返回重新中断，
         * 如果未中断，则返回0。
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ?
                    (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                    0;
        }

        /**
         * Throws InterruptedException, reinterrupts current thread, or does nothing,
         * depending on mode.
         * <p>
         * 抛出InterruptedException、重新中断当前线程或不执行任何操作，这个具体取决于模式。
         */
        private void reportInterruptAfterWait(int interruptMode)
                throws InterruptedException {
            if (interruptMode == THROW_IE) {
                throw new InterruptedException();
            } else if (interruptMode == REINTERRUPT) {
                selfInterrupt();
            }

        }

        /**
         * Implements interruptible condition wait.
         * <p>
         * 实现可中断条件等待。
         *
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled or interrupted.
         * <li> Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        @Override
        public final void await() throws InterruptedException {
            if (Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) {
                    break;
                }

            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE) {
                interruptMode = REINTERRUPT;
            }

            if (node.nextWaiter != null) {
                // clean up if cancelled
                unlinkCancelledWaiters();
            }

            if (interruptMode != 0) {
                reportInterruptAfterWait(interruptMode);
            }

        }

        /**
         * Implements timed condition wait.
         * <p>
         * 实现定时条件等待。
         *
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        @Override
        public final long awaitNanos(long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }

                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) {
                    break;
                }

                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE) {

                interruptMode = REINTERRUPT;
            }
            if (node.nextWaiter != null) {

                unlinkCancelledWaiters();
            }
            if (interruptMode != 0) {
                reportInterruptAfterWait(interruptMode);
            }

            return deadline - System.nanoTime();
        }

        /**
         * Implements absolute timed condition wait.
         * <p>
         * 实现指定时间条件等待。
         *
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        @Override
        public final boolean awaitUntil(Date deadline)
                throws InterruptedException {
            long abstime = deadline.getTime();
            if (Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (System.currentTimeMillis() > abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) {
                    break;
                }

            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE) {

                interruptMode = REINTERRUPT;
            }
            if (node.nextWaiter != null) {
                unlinkCancelledWaiters();
            }

            if (interruptMode != 0) {
                reportInterruptAfterWait(interruptMode);
            }

            return !timedout;
        }

        /**
         * Implements timed condition wait.
         * <p>
         * 实现定时条件等待
         *
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        @Override
        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold) {
                    LockSupport.parkNanos(this, nanosTimeout);
                }
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) {
                    break;
                }
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE) {
                interruptMode = REINTERRUPT;
            }

            if (node.nextWaiter != null) {

            }
            unlinkCancelledWaiters();

            if (interruptMode != 0) {
                reportInterruptAfterWait(interruptMode);
            }

            return !timedout;
        }

        //  support for instrumentation 对仪器的支持

        /**
         * Returns true if this condition was created by the given synchronization object.
         * <p>
         * 如果此条件是由给定的同步对象创建的，则返回true。
         *
         * @return {@code true} if owned
         */
        final boolean isOwnedBy(MyAbstractQueuedSynchronizer sync) {
            return sync == MyAbstractQueuedSynchronizer.this;
        }

        /**
         * Queries whether any threads are waiting on this condition.
         * Implements {@link MyAbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
         * <p>
         * 查询是否有线程在此条件下等待。
         * 实现AbstractQueuedSynchronizer.hasWaiters(ConditionObject)
         *
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        protected final boolean hasWaiters() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }

            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    return true;
                }

            }
            return false;
        }

        /**
         * Returns an estimate of the number of threads waiting on this condition.
         * Implements {@link MyAbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
         * <p>
         * 返回在此条件下等待的线程数的估计值。
         * 实现 AbstractQueuedSynchronizer.getWaitQueueLength(ConditionObject)
         *
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    ++n;
                }

            }
            return n;
        }

        /**
         * Returns a collection containing those threads that may be waiting on this Condition.
         * Implements {@link MyAbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
         * <p>
         * 返回包含可能在此条件下等待的线程的集合
         * 实现AbstractQueuedSynchronizer.getWaitingThreads(ConditionObject)
         *
         * @return the collection of threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            }
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null) {
                        list.add(t);
                    }

                }
            }
            return list;
        }
    }

    /**
     * Setup to support compareAndSet. We need to natively implement this here:
     * For the sake of permitting future enhancements, we cannot explicitly subclass AtomicInteger,
     * which would be efficient and useful otherwise.
     * So, as the lesser of evils, we natively implement using hotspot intrinsics API.
     * And while we are at it, we do the same for other CASable fields
     * (which could otherwise be done with atomic field updaters).
     */

    // private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final Unsafe unsafe = reflectGetUnsafe();
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;


    public static Unsafe reflectGetUnsafe() {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            return (Unsafe) field.get(null);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
    }

    static {
        try {
            stateOffset = unsafe.objectFieldOffset
                    (MyAbstractQueuedSynchronizer.class.getDeclaredField("state"));
            headOffset = unsafe.objectFieldOffset
                    (MyAbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset
                    (MyAbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset
                    (Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset
                    (Node.class.getDeclaredField("next"));

        } catch (Exception ex) {
            throw new Error(ex);
        }
    }

    /**
     * CAS head field. Used only by enq.
     * <p>
     * CAS head
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * CAS tail field. Used only by enq.
     * <p>
     * CAS tail
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS waitStatus field of a node.
     * <p>
     * CAS waitStatus
     */
    private static final boolean compareAndSetWaitStatus(Node node,
                                                         int expect,
                                                         int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                expect, update);
    }

    /**
     * CAS next field of a node.
     * <p>
     * CAS 下一个
     */
    private static final boolean compareAndSetNext(Node node,
                                                   Node expect,
                                                   Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }
}
